Engineering specifications for O-ring seal surfaces are well documented. However, when seal surfaces are located
on asymmetrically loaded vacuum end-plates, consideration must be given not only to surface finish and
flatness, but also to load-induced deflections. When deflections are significant, O-ring compression can relax
and potentially cause vacuum leaks. Large vacuum systems, such as the 9000 cubic foot system at the Navy
Prototype Optical Interferometer (NPOI), cannot afford costly vacuum leaks due to improper end-plate design.
The NPOI employs vacuum end-plates that serve both as structural members, and as vacuum system entrance
and exit ports for stellar light. These ports consist of vacuum components attached directly to the end-plate via
static O-ring sealing techniques. Optical geometry dictates off-center port locations, which create asymmetric
end-plate loading. This paper details the behavior of a 22 inch diameter, multi-port, end-plate for the NPOI Fast
Delay Line subsystem. In depth CAD modeling and finite element analysis techniques were used to determine
load-induced stress distributions and deflections in the end-plate. After several design iterations, an end-plate
design was substantiated that maintains vacuum seal integrity under loading, exhibits a conservative factor of
safety, and is readily manufacturable.